As optical fiber communications channels increasingly replace metal cable and microwave transmission links, integrated optical devices for directly processing optical signals become increasingly important. A useful approach to optical processing is through the use of integrated glass waveguide structures formed on silicon substrates. The basic structure of such devices, commonly called silica optical circuits, is described in C. H. Henry et al., "Glass Waveguides on Silicon for Hybrid Optical Packaging", 7 J. Lightwave Technol., pp. 1530-1539 (1989) which is incorporated herein by reference. In essence a silicon substrate is provided with a base layer of SiO.sub.2, and a thin core layer of doped silica glass is deposited on the oxide. The core layer can be configured to a desired waveguide structure-typically 5-7 micrometers wide-using standard photolithographic techniques, and a layer of doped silica glass is deposited on the core to act as a top cladding. Depending on the precise configuration of the waveguide, such devices can perform a wide variety of functions such as beam splitting, tapping, multiplexing, demultiplexing and filtering.
One shortcoming of such devices, however, is birefringence induced in the waveguide core by compressive strain. The compressive strain is due to differential thermal expansion between the silicon and the silica. The effect of the resulting birefringence is that different polarization modes of transmitted light are presented with different effective indices of refraction. Specifically, the transverse magnetic mode (TM) encounters a greater index than does the transverse electric (TE) mode. The dispersive effect is further aggravated by curves in the waveguide. When traversing a curve, optical modes are shifted radially outward. A mode loosely bound to the waveguide core (TM) will experience a greater outward shift than a mode more tightly bound (TE) with the consequence that the loosely bound mode has a greater optical path length and phase.
Elimination of the resulting birefringence has long been recognized as necessary for high performance optical devices. Indeed a variety of elaborate schemes have been proposed for compensating such birefringence. One method employs a half-wave plate inserted in the middle of a waveguide grating multiplexer to rotate the polarization by 90.degree.. See H. Takahashi et al., "Polarization-Insensitive Arrayed-Waveguide Multiplexer on Silicon", Opt. Letts., 17 (7), p. 499 (1992). This approach leads to excessive loss. Another approach is to deposit on the waveguide a thick layer (six micrometers) of amorphous silicon. But the silicon layer must then be actively trimmed with a high power laser. See M. Kawatchi, et al, "Laser Trimming Adjustment of Waveguide Birefringence In Silica Integrated Optic Ring Resonators", Proc. CLEO '89, Tu J. 17 (April 1989). Yet a third approach is to etch grooves alongside the waveguide to release strain. This approach requires deep grooves on the order of 60 micrometers deep. See M. Kawatchi et al, "Birefringence Control in High-Silica Single-Mode Channel Waveguides in Silicon", Proc. OFC/I00C '87, Tu Q 31 (Jan. 1987). Accordingly, strain-induced birefringence is clearly a problem in silicon based integrated optical devices.